Scrapie associated fibrils (SAF) are infection specific markers of unconventional slow virus diseases. Similar biological and biophysical properties including coisolation in several purification procedures indicate a strong association betwewn SAF and the infectiuous agent. These studies, however, have not established identity nor identified the components present in purified infectious SAF fractions which are essential for infectivity. This proposal will further purify SAF employing methodologies and immunological reagents developed during the previous granting period and further establish the relatinoship between SAF and infectivity. Components present in purified infectious fractions will be analyzed for their relationship to the infectious agent. Specific scrapie strain and host influence on the protein components of SAF will be investigated using monoclonal antibodies as epitope markers to analyze a variety of scrapie strains passaged in different host species. Our unique ability to examine this range of scrapie strain combinations will afford insight into specific scrapie strain induced modifications involved in structrual and conformational properties of SAF and their relationship to the infectious agent. The existence of biologically and pathologically defined scrapie strained implies the presence of an "informational molecule" in addition to modified host protein (PrP) as a component of the infectious agent. This molecule is likely to be a small regulatory RNA. Probes are being generated to the central conserved region of viroid RNA, the consensus sequence of intron group 1 and to unique RNA species associated with highly purified SAF. These probes in conjunction with electron microscopic and nucleic acid binding studies will identify specific nucleic acids associated both with SAF and the infectious agent. Generation of an expression vector system will serve as a first step towards correlating components of SAF, including processing or modification of PrP, with their essential role in infectivity. This multi-disciplined approach will lead to an understanding of the role of SAF in agent replication and stability, pathogenesis and their unique association with this unusual class of infectious agents.